Download Chapter 7: Earthquakes

Chapter 9: Earthquakes
9.1: Earthquakes occur along faults
9.2: Earthquakes release energy
9.3: Earthquake damage can be
reduced
9.2 Earthquakes release energy
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Terms:
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faults,
plate boundaries
stress
earthquake
All earthquakes occur along ______
The force exerted when an object pushes, pulls, or
presses against another object is called _______
Most faults are located along _______
Earthquakes release energy
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Energy from earthquakes travel through Earth
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Ripple of rock in pond, but in all directions
Energy travels as seismic waves: vibrations caused
by earthquakes
Earthquakes start beneath Earth’s surface
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Focus: point underground where rocks first begin to
move
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Seismic waves travel outward from the focus
Epicenter: point on Earth’s surface directly above the
focus
If equal strength earthquakes occur, the more
shallow the focus results in greater damage
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Depth is related to the direction in which the plate
move
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Pulling apart: shallow: new crust that forms is thin
Subduction zones: wide range of depths, anywhere
along sinking plate
CA video
Waves and Energy
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All waves (sound, seismic, light) carry energy
from place to place
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As a wave moves through a material, particles of
the material move out of position temporarily,
causing the particles next to them to move:
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Energy moves through the material, matter does not
(*light is different)
October 17th earthquake in San Francisco: shook
the stadium around for 15 seconds
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20 minutes later the seismic waves reached the other
side of the Earth: detect only by sensitive scientific
instruments
Three types of waves
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Each type moves through material differently
Can reflect, or bounce, off boundaries
between different layers
Can bend as pass from one layer to another
Scientists learn about Earth’s layers by
studying the paths and speeds of seismic
waves traveling through Earth
waves video
Three types of waves
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Primary (P waves)
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the fastest seismic waves First to reach any
particular location after an earthquake
Travel through Earth’s crust at an average speed
of 5 km/s (3 mi/s)
Can travel through solids, liquids, gases
As they travel through a material the particles are
slightly pushed together and pulled apart
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Buildings experience this push and pull as p waves pass
through the ground
Three types of waves
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Secondary waves (S waves)
 the second waves to reach a location after an earthquake
 Originate at the same time as P waves, but travel at half the
speed
 As they pass through a material, the material’s particles are
shaken up and down or side to side
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can travel through rock but not liquids or gases
Primary waves alter the material density and volume slightly
 Particles are pushed and pulled in the direction the waves travel
Secondary waves alter the material’s shape
 Liquids and gases have no definite shape
 Particles move at a right angel to the direction the waves travel
When scientist learned S waves cannot pass through the earth’s
outer core they realized it was not solid!
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This rocks buildings back and forth as they pass
Three types of waves
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Surface waves
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Move along Earth’s surface, not through the
interior
Make the ground roll up and down or shake from
side to side
Cause the largest ground movement and most
damage
Slowest type of seismic wave
As depth increases, motion of the particles
decreases
Types of waves
Name
Primary
Secondary
Surface
Speed
Fastest
Medium
Slowest
Location
Earth’s
interior
All
Earth’s
interior
Solids
Earth’s
surface
_____
Push/pull
Up/down;
side/side
Some
Up/down;
side/side
Most
Type of
material
Type of
movement
Damage
Some
Seismic waves can be
measured
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Seismograph: an instrument that
constantly records ground
movements
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Separate ones are needed to record
side-to-side movements and up-anddown movements
Side-to-side: uses a heavy weight
attached to a wire, which remains still as
the ground moves beneath it
Up-and-down: uses a heavy weight
hanging from a spring, which remains
almost still as the spring absorbs the
movement by getting longer or shorter
Can detect movements as little as one
hundred-millionth of a centimeter
(0.000001cm)
Locating an Earthquake
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1.
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Need seismographs from at
least three stations
Scientists find the difference
between the arrival times of the
primary and secondary waves
at each of the three stations
The time difference is used to
determine the distance of the
epicenter from each station
(the greater time, the father
away)
A circle is drawn around each
station, with a radius
corresponding to the
epicenter’s distance from that
station. Where the three
circles meet is the epicenter
If you know the speed of S and
P waves, and you know the
time it takes for their arrival,
you can determine distance
Speed = distance/time
While the speeds vary, the
ratio of the speeds do not! Just
multiply the S-minus-P (S-P)
time, in seconds, by the factor
8 km/s to get the approximate
distance in kilometers.
Locating an Earthquake
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Seismographs can also be used to locate the focus
of an earthquake
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Can study waves that have reflected off boundaries inside
Earth
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Record the time when
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Can help determine the earthquake’s depth
the first primary wave arrives, by a direct path
and also the first reflected primary wave arriving, which first
reflects off the surface before reaching the station
The difference in arrival time indicates the depth of the focus
Also used to determine earthquakes’ magnitude
(strengths)
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More energy an earthquake releases, the greater the
ground movement recorded